I. Field of the Invention
This invention relates to a whipstock starter mill for redirecting a drill bit or cutter at an angle from the well casing and, in particular, to a tattletale assembly associated with the whipstock and mill for creating an in-line pressure drop when the mill has reached a predetermined depth along the whipstock
II. Description of the Prior Art
Whipstocks have long been used in drilling to direct a drill bit or cutter at an angle from the set casing. The whipstock may be set within the casing above the bottom of the well bore using a packer which may be run separately or together with the whipstock in one trip. In the one-trip packstock mill, the mill tool, whipstock and packer are run into the hole integrally and the packer is set at the desired depth. With the packstock set, the mill is detached from the whipstock and the milling operating is initiated. The mill travels down the angled surface of the whipstock to engage and mill through the casing wall. However, because of the nature of the mill, the casing material and the whipstock, care must be taken to mill only through the casing wall. If the mill travels too far through the casing, the surrounding formation material will cause the mill to mill into the whipstock eventually destroying either the mill or the whipstock. If the casing is not milled completely through, the cutter or drill bit subsequently run into the hole will not travel in the desired direction.
In the past known whipstock operations, the depth of the mill has been largely determined by the operator at the surface. However, such determinations relied on some guess work since deviations in the casing etc. at times belied the actual depth. Because the depth of the mill was largely determined by the total travel into the hole, miscalculations have occurred.